Hyperspectral imaging is an extremely powerful and broadly applied technique of optical remote sensing. It consists of imaging an object or scene in a multitude of contiguous wavelength intervals. As distinct from multi-spectral imaging, where the wavelength intervals are typically defined by a relatively small number (e.g., <12) of discrete optical bandpass filters, hyperspectral imaging employs a larger number of finer spectral intervals in order to more reliably detect and discriminate between the unique spectral characteristics of natural and artificial materials.
As a tool of remote sensing, for example in geology or astronomy, hyperspectral imaging is generally practical only in wavelength regions where the earth's atmosphere is transparent over ranges of many kilometers and not affected by absorption from atmospheric constituents such as carbon dioxide and water vapor. These regions include the visible and near-infrared (VNIR: 0.4-1.0 μm), short-wave infrared (SWIR: 1.0-2.5 μm), mid-wave infrared (MWIR: 3.0-5.0 μm), and long-wave infrared (LWIR: 8.0-14.0 μm). Depending on the mission and phenomenology of interest, it is common to divide one of these broad spectral regions into 10s or 100s of sampled spectral sub-intervals. Typical numbers range from 32 to 512. These modest spectral resolutions are most practically achieved with spectrometers based on dispersive prism or diffraction grating elements.
The preferred detector for modern hyperspectral instruments is the two-dimensional array of discrete, solid-state electronic sensing elements. Array detectors are available and optimized for each of the wavelength regions and include silicon charge-coupled devices (CCDs) for the VNIR, InGaAs, InSb, and HgCdTe for the SWIR, HgCdTe and InSb for the MWIR, and HgCdTe, extrinsic doped silicon (e.g., Si:As), and microbolometer arrays for the LWIR. Individual element (pixel) sizes range from 8 to 75 μm, and array dimensions presently range from 128 to 1024 elements in each dimension and are not constrained to square formats.
Two common methods of forming hyperspectral images of extended scenes are pushbroom and whiskbroom scanning. Both methods share the attribute of defining an instantaneous spatial area of interest by a long, narrow aperture mask (or slit) at the focus of an imaging system. Typical aspect ratios for slits range from 1:128 to 1:1024. Light from the scene passing through the slit is then spectrally dispersed perpendicular to the long dimension (length) of the slit and reimaged onto a two dimensional detector array. If the slit is parallel to the rows of the detector array, when read out, the rows of the array are then contiguous spectral images of the slit. Alternatively, the columns of the array represent the spectra of each spatial sample in the slit as defined by the array pixels. If the projection of the slit on the object or scene is advanced by its narrow dimension (width) each time the detector array frame is read out, a complete spectral map of every point in a two-dimensional scene is built up over time. Pushbroom and whiskbroom scanning are different in that for the former, the motion of the slit is produced by the motion of the platform (e.g., aircraft or satellite) carrying the hyperspectral imaging sensor, whereas for the latter, the scan direction is perpendicular to the platform motion and is typically accomplished by a scanning mirror directing the sensor line of sight within a defined field of regard.
For a given uniform and continuous scene, the signal collected by a detector element in a hyperspectral imager is proportional to the area of the detector, the angular extent (in two-dimensions, or steradians) of the imaging beam focused on the detector, the spectral sampling interval, and the time the signal is integrated between readouts of the detector array. The product of the first two quantities is equivalent to a metric called the throughput (also etendue or AΩ product). For high-performance hyperspectral imaging instruments intended to cover large areas with both fine spatial and spectral resolutions, obtaining adequate signal levels in a short integration time depends on achieving the largest possible throughput in the spectrometer instrument, equivalent to a fast focal ratio (f/#) or large numerical aperture (N.A.). Focal ratios of at least f/2.5 (N.A. >0.2) are necessary, and some applications benefit from focal ratios as fast as f/0.7 (N.A. 0.86).
In addition to large numerical aperture, spectrometers for hyperspectral remote sensing must have good image quality relative to the size of the detector element. For reliable processing and interpretation of the hyperspectral data, they should also have low image distortions such that the length of the slit image should not change with wavelength (keystone) and the dispersed position of the slit image for a given wavelength should not change with position along the slit (smile). These distortions should be controlled to a small fraction (e.g., < 1/10) of a pixel dimension.
Finally, spectrometers intended for aircraft or satellite use, where volume and mass are often highly constrained, must be as compact as possible. This is particularly true of instruments operating at infrared wavelengths, where the entire instrument is cooled to cryogenic temperatures and demands on cooling resources increase dramatically with instrument volume.
For the above set of requirements, one of the best starting designs or solutions is the Dyson spectrometer. It has excellent image quality and low distortion at very large numerical apertures, while at the same time being very compact.
Dyson first demonstrated that a simple concentric arrangement of a piano convex refracting element (lens) and concave mirror would be free of all Seidel aberrations at the design wavelength and center of a field imaged at 1:1 magnification. J. Dyson, “Unit magnification optical system without Seidel aberrations,” J. Opt. Soc. Am. 49, 713-716 (1959). Higher order aberrations being more sensitive to field than to numerical aperture, Dyson suggested the form could be applied to photolithography of very fine features of limited spatial extent at high numerical aperture, an observation that would subsequently find application in the developing field of microlithography. Mertz later proposed that the Dyson principle could form the basis for a very high throughput spectrometer, and demonstrated a numerical aperture of 0.66 (f/0.7) over a limited field and wavelength range in the ultraviolet using a holographically generated grating in place of the Dyson's concave mirror. L. Mertz, “Concentric spectrographs,” Appl. Opt. 16, 3122-3124 (1977). Referring to C. G. Wynne, “Monocentric telescopes for microlithography,” Opt. Eng. 26, 300-303 (1987), Wynne continued to refine the Dyson form for microlithography in the visible and ultraviolet, correcting the field with cemented refractive elements, but development of the Dyson for spectrographic applications languished until the early 1990's, when it was revived for space applications. D. R. Lobb, “Theory of concentric designs for grating spectrometers,” Appl. Opt. 33, 2648-2658 (1994); D. R. Lobb, “Imaging spectrometers using concentric optics,” Proc. SPIE 3118, 339-347 (1997). More recently, Mouroulis and Green considered Dyson designs for visible spectrometry. P. Mouroulis and R. O. Green, “Optical design for imaging spectroscopy,” Proc. SPIE 5173, 18-25 (2003). However more attention and development has gone into the Offner form and derivatives (Lobb, Mouroulis and Green), which are also based on concentric principles and have some competitive advantages for systems with more modest f/ratios.
In all of these designs, the slit and detector must be in close proximity to the rear face of the thick refracting element characteristic of the Dyson form. This aspect of the design makes it very difficult to mount detector arrays, which may be protected by windows of significant thickness, be recessed into chip carriers, or be surrounded by delicate wire bonds. Previous designs have attempted to ameliorate this drawback by bonding additional elements to the refractive element. However, these techniques will not work for cryogenic infrared instruments, where bonding adhesives will neither transmit nor survive extreme thermal cycling. The best way to simplify focal plane integration is to increase the gap between the focal plane and the rear surface of the refracting element. However, doing so rapidly degrades image quality by introducing spherical aberration, particularly for very high throughput systems. As focal ratios shrink from f/2.5 to f/1.0, the permissible gaps shrink from one or two millimeters down to a fraction of a millimeter, all of which are impractical to implement in practice.
Spherical aberration is the principal driver keeping the image surface close to the rear surface of the Dyson lens. For a given system, the geometric image size due to spherical aberration increases linearly with the image plane relief and also as the cube of the f/ratio as the throughput is increased. At f/1, image quality is unacceptable with only a few hundred microns of relief, not enough to be useful. Thus, although Dyson spectrometers are potentially very attractive because of their compactness, excellent image quality, and high throughput (fast focal ratio), their one drawback in practical implementation is the difficulty in mounting focal planes in close proximity to the refracting element.
It would be helpful to be able to provide a Dyson spectrometer apparatus or system which accommodates a greater distance between the plane containing the entrance aperture (e.g., slit) and detector (e.g., focal plane array) and the rear surface of the Dyson lens without incurring an unacceptable penalty in respect to spherical aberration.
It would be helpful to be able to provide a long-wave infrared (LWIR) imaging spectrometer which accommodates a greater distance between the plane containing the entrance aperture (e.g., slit) and detector (e.g., focal plane array) and the rear surface of the Dyson lens without incurring an unacceptable penalty in respect to spherical aberration, and without the use of cemented optical interfaces.